Printed electronics represents a new generation of large-area, flexible, ultra-low-cost, and disposable electronics such as backplane electronics for flat-panel displays, biosensors and imagers, radio frequency identification tags, smart labels and packaging, to name a few. To enable substantially lowered manufacturing costs, the electronic arrays and circuitries for printed electronics are to be produced by low-cost manufacturing processes such as, for example, printing using graphic arts printing techniques instead of conventional costly photolithographic processes. Specifically, printing electronic arrays and circuitries in a roll-to-roll manner will greatly increase throughput and thus significantly reduce manufacturing costs owing to reduced process steps and economies of scale of production. Low-cost electronic arrays and circuitries can also be cost-effectively printed in large formats, thus making large-area electronic devices economical. In addition, printed electronics can be fabricated on flexible plastic substrates, rendering the resulting electronic devices structurally flexible and bendable as well as being mechanically robust.
The building blocks for printed electronic arrays and circuitries are organic thin-film transistors (OTFTs) whose channel semiconductors are based on organic semiconductors. For most high-value electronic applications (e.g., in flat-panel displays, RFID, etc.), the field-effect mobility and current on/off ratio of OTFTs of respectively at least about 2 cm2V−1s−1 and 105 and beyond would be highly desirable. Certain small molecular organic fused-ring aromatic compounds and organic conjugated polymers, particularly those with alternating electron donor and electron acceptor moieties on their backbones, have captured immense interest as semiconductors of choice for OTFTs as they have exhibited reasonably high field-effect mobility and current on/off ratio that appear to meet application requirements. Notwithstanding these remarkable achievements in materials performance, progress towards OTFT technology transition from laboratory to marketplace has lagged behind. One of the major stumbling blocks to practical adoption has been the complications associated with the solution processing of organic semiconductors. For organic small molecular fused-ring aromatic semiconductors, the properties of the resulting channel semiconductor films formed from these compounds via solution fabrication are particularly sensitive to the nature and manner of semiconductor deposition. This is because the crystallinity, which affects the field-effect properties of deposited semiconductor films, is dependent on the rate of solvent evaporation. Generally, slow evaporation leads to higher crystallinity in the semiconductor films and thus better OTFT performance characteristics, while fast evaporation yields poor and non-uniform semiconductor films of low crystallinity and poor OTFT properties. Accordingly, great variations in OTFT properties result when small molecular semiconductors are processed by solution deposition. For polymer semiconductors, the solution or ink preparation for coating or printing channel semiconductor layers requires the polymer semiconductor to have a good solubility in the processing solvent. Unfortunately, most high-mobility polymer semiconductors for OTFTs are polar polymers with relatively high molecular weights, and thus limited solubility in common processing solvents. While these polymers may be dissolved in the processing solvents with heating, their solutions tend to gel or exhibit solid precipitation or inhomogeneity when the solutions are cooled down to room temperature. This has caused severe complications in fabricating or printing the channel semiconductors for OTFTs. The use of lower-molecular weight polymer semiconductor materials, which possess reasonably good solubility in processing solvents, affords low mobility. This is because the charge transport efficacy of polymer semiconductors degrades rapidly with decreasing molecular weight. High molecular weights appear to facilitate molecular self-assembly of polymer semiconductors, leading to higher crystalline orders which are crucial to charge carrier transport efficiency and thus field-effect mobility of OTFTs.
One of the possible approaches to circumventing the afore-mentioned fabrication complications for both small molecular and polymer semiconductors is to utilize a polymer to mediate molecular self-assembly of semiconductors. This method permits utilization of a lower concentration of semiconductor in the fabrication solution, thus yielding a more stable solution. The polymer-in-solution affords a fluid medium for the semiconductor molecules to move around and self-assemble to higher degrees of molecular ordering. Through this polymer-mediated molecular self-assembly, the dissolved small molecular or polymer semiconductors including low-molecular weight polymer semiconductors, may be processed into semiconductor films of higher molecular orders, affording significantly enhanced charge transport efficacy, and thus superior OTFT properties.
A prior-art example for the attainment of highly aligned crystalline films from small molecular semiconductor in a polystyrene blend has been reported in “Ultra-high mobility transparent organic thin film transistors grown by an off-centre spin-coating method”, Nature Communications, Vol 5:3005 (2014) DOI: 10.1038/ncomms4005 |www.nature.com/naturecommunications, the disclosure of which is hereby incorporated by reference. In this process, the small molecular semiconductor and polystyrene are dissolved in a solvent and fabricated into a polystyrene-semiconductor dual-layer film using an off-center spin-coating technique. The small semiconductor molecules phase separate from the polystyrene in a vertical manner and the centrifugal force exerted on the semiconductor in the off-center position forces the small semiconductor molecules to align into a highly crystalline film. Another prior art example involves a polymer material such as regioregular poly(3-hexythiophene) as described in “Organic Thin-film Transistors Based on Polythiophene Nanowires Embedded in Insulating Polymer”, Advanced Materials, Vol. 21, pp. 1349-1353 (2009), the disclosure of which is hereby incorporated by reference. Crystalline nanowires with improved charge transport properties have been grown from this polymer semiconductor at very low loadings in polystyrene blends with the help of a marginal solvent such as methylene chloride. Similarly, high-mobility, single-crystal nanowires of a polymer semiconductor has been produced from its dilute solution in a marginal solvent, see for example, “High Aspect Ratio Conjugated Polymer Nanowires for High Performance Field-Effect Transistors and Phototransistors”, American Chemical Society Nano, Vol. 9, pp. 5264-5274 (2015), the disclosure of which is hereby incorporated by reference. However, all these prior-art solution procedures for achieving high molecular orders to enhance charge transport properties are not amenable to conventional coating or printing processes, thus rendering their use in fabricating low-cost OTFTs for practical applications impractical. Achievement of high molecular orders for organic semiconductors via scalable solution processes to produce low-cost OTFTs with enhanced charge transport properties has not been reported.
Accordingly, it is an objective of the present invention to provide a solution or ink composition which permits fabrication of a channel semiconductor via scalable solution processes to enable OTFTs with enhanced transistor performance properties.
Another objective of the present invention is the provision of a scalable solution process for fabricating a channel semiconductor for high-performance OTFTs which are suitable for low-cost electronic applications.
A further objective of the present invention is to provide a stable solution or ink composition comprising a semiconductor and a mediating polymer or polymers in a suitable solvent, wherein said solution or ink can be utilized in the fabrication of a channel semiconductor via high-throughput processes for high-performance OTFTs.